1. Field of the Invention
This invention relates to a cryogenic vessel and, more specifically, to an over-pressurization protection system for a cryogenic vessel that is part of a cryogenic liquid delivery system.
2. Background Information
Cryogenic liquids, such as liquid argon, natural gas (LNG), nitrogen, oxygen, CO2, hydrogen and the like, are substances that normally exist as gasses, but are liquids at cold temperatures. Special vessels and systems must be used to store and transfer cryogenic liquids because of difficulty in maintaining the extremely cold temperatures. Such vessels typically include a double walled vessel defining a storage space and having a vacuum in the annular space. The vessel has multiple lines extending into the storage space, including a fill line. If the vessel is not vented sufficiently, pressure within the vessel is increased during filling operations when the cryogenic liquid is pumped into the vessel through the fill line. To prevent damage to the vessel, it is desirable to maintain the vessel internal pressure within the design limits of the vessel.
To date, cryogenic vessels include a variety of safety features to prevent an over-pressurization situation from causing permanent damage to the cryogenic vessel. Such safety features include relief valves structured to vent gas to the atmosphere and burst disks structured to be ruptured in severe over-pressurization situations. The valves may have operated mechanically or may have been linked to a computer control system which had pressure monitoring devices and was structured to open the valves when an over-pressurization condition occurred. While these devices are sufficient to protect the vessel from damage, each has disadvantages. For example, the relief valves vent the gas to the atmosphere causing a loss of product. Further, venting to the atmosphere may be undesirable or dangerous in certain situations, such as venting LNG in an enclosed area. Burst disks rupture when a set pressure is exceeded. As such, al product within the vessel is lost and the burst disk must be replaced. Additionally, burst disks also vented gas to the atmosphere. Generally, these devices prevent damage to the vessel when an over-pressurization situation occurs, but do not avert over-pressurization situation from occurring.
Additionally, the cryogenic vessel may include a delivery system having a recirculation system. The delivery system includes a delivery line extending into the storage space and structured to deliver the cryogenic liquid to a bulk vessel or a cylinder. The delivery system may include a pump and a pressure building circuit. The recirculation system included a recirculation line having a recirculation valve thereon. The recirculation line extended between, and was in fluid communication with, the delivery line and the fill line. During the delivery of the cryogenic liquid to a bulk vessel, the recirculation valve closed so that the entire flow was directed to the bulk vessel. During the delivery of the cryogenic liquid to a cylinder, the recirculation valve opened so that any excess flow was returned to the cryogenic vessel. For example, the cryogenic vessel may be a movable bulk storage vessel, such as a delivery truck used to transport fuel to filling stations or other stationary bulk tanks. The cryogenic vessel is structured to fill both stationary bulk tanks, such as the filling station, as well as smaller, portable cylinders, or even a fuel tank on a vehicle.
There is a need, therefore, to have cryogenic vessel with an over-pressurization protection system.
There is a further need to reduce the cost of any such system by utilizing components already existing on the cryogenic vessel delivery system.